It is desirable in automated chemical analyzers to store reagents in a resealable container. Such a container allows access by perforating the container's lid for removing fluid, and after fluid withdrawal automatically reseals the perforation in the container's lid. This resealing action, known as self healing is essential to controlling reagent integrity over time. There are many known containers of this type. One such container is sold for use with the Dimension.RTM. Clinical Chemistry System by E. I. du Pont de Nemours and Co., Wilmington, DE 19898. Another such container and lid construction is described in a patent application entitled "Lid Structure" Ser. No. 07/237,011, filed Aug. 16, 1988 by DeBenedictis now U.S. Pat. No. 4,935,274.
Simultaneously however, the resealable lid/container structure which lengthens reagent life, also inhibits the ability of an automated chemical analyzer to accurately remove fluid from the container. This happens because the flexible or elastomeric material used for the lid seals tightly around the pipetting probe or needle when the probe is inside the container. When fluid is removed, a vacuum is produced because the seal does not allow air to enter. This vacuum could adversely affect pipetting performance.
This vacuum condition lasts over a period of time, depending on many variables, but has been observed to last for days. So the effect is felt on the immediate aspiration as well as those that which may be performed in the future. The effect of repeated aspirations from the same container can also be additive. Thus each time fluid is removed, the vacuum become greater and greater.
Accuracy is consistently affected the same way. When the reagent pump attempts to draw 100% of the quantity desired, what is actually withdrawn from the container is less than 100%, usually in the 90 to 95% range of that desired. This is what one would expect; the reagent pump is applying a vacuum to draw fluid in while the vacuum in the container is tending to pull the fluid out. The pump actually gets less than that desired.
Precision of the reagent delivery is also affected by the vacuum. More spread or greater imprecision is observed with the container vacuum. This imprecision is attributed to, among other, gas in the fluid line, varied vacuum levels producing different effects, and bouncing of the fluid miniscus when withdrawing the probe from the container. Such variables are difficult if not impossible to control, so a means of eliminating the vacuum is needed.
This is a recognized problem and various techniques have been employed in the prior art for reducing the effects of this vacuum phenomena that's created within sealed containers. Thus, Gustavsson in U.S. Pat. No. 4,673,404 discloses an adapter device for venting and pressure balancing a sealed vessel. A vent needle pierces the closure of the sealed vessel allowing the vessel to vent through a filter. An aspiration needle may enter the vessel through the sealing member and the vessel closure to aspirate fluid from the vessel. While this approach aids in solving the problem, it also creates unnecessary punctures in the cap which can cause loss of fluid from the vessel in the container due to evaporation.
Another approach was taken by Averette in U.S. Pat. No. 4,815,325. As may be seen in FIG. 4A, Averette uses a coaxial probe to aspirate fluid from a sealed vessel. After the probe penetrates the sealing member of the vessel, fluid from the vessel may be aspirated through the inner tube of the coaxial probe to the aspiration tubing. During aspiration, air is vented into the vessel through the vent tubing and down through the outer annulus of the probe to the opening in the side of the coaxial probe. While this is a satisfactory alternative, where cross contamination is a concern the carryover problems can be severe.
Ringrose et al. in U.S. Pat. No. 3,872,730 discloses a device for sampling from a closed blood tube such as a "Vacutainer" tube. A dual needle probe penetrates the closure of the tube, allowing the first needle to vent the interior of the tube to atmosphere and the second needle to aspirate fluid from the tube. Here again, the double penetration which occurs each time a sample is taken leads to the more rapid deterioration of the integrity of the seal of the container.
Finally, Uffenheimer U.S. Pat. No. 4,756,201 discloses a device for sampling from a closed blood tube such as a "Vacutainer" tube. Referring to FIG. 1 of Uffenheimer, when the probe first enters the tube, through the closure, the shear valve is positioned such that ambient air in the equilibration chamber is allowed to vent into the tube. After venting the shear valve is repositioned to allow the pump to aspirate from the tube.
This arrangement, which requires air in the system from the valve down to the probe, to eliminate contaminating the container with fluid is not always a desirable solution to the problem.